This Ecma Standard specifies the signalling protocol for the
support of the Call priority interruption (SS-CPI) and Call Priority
Interruption Protection (SS-CPIP) supplementary services at the
Q reference point between Private Integrated services Network
eXchanges (PINXs) connected together within a Private Integrated
Services Network (PISN).

NOTE 1
This edition of this Standard does not apply to calls using the
circuit-mode multiple rate bearer service.

SS-CPI allows a call request for a priority call to proceed successfully
in the case that there is no user information channel available.
This is accomplished by force releasing an established call of
lower priority.

SS-CPIP allows for the protection of calls against interruption
from priority calls.

Service specifications are produced in three stages and according
to the method specified in ETS 300 387. This Ecma Standard contains
the stage 3 specification for the Q reference point and satisfies
the requirements identified by the stage 1 and stage 2 specifications
in ECMA-263.

The signalling protocol for SS-CPI(P) operates on top of the
signalling protocol for basic circuit switched call control, as
specified in ECMA-143, and uses certain
aspects of the generic procedures for the control of supplementary
services specified in ECMA-165.

This Standard also specifies additional signalling protocol requirements
for the support of interactions at the Q reference point between
SS-CPI(P) and other supplementary services and ANFs.

NOTE 2
Additional interactions that have no impact on the signalling
protocol at the Q reference point can be found in the relevant
stage 1 specifications.

This Ecma Standard is applicable to PINXs that can interconnect
to form a PISN.

The protocol defined in this Standard forms part of the PSS1
protocol (informally known as QSIG).